How to calculate barometric pressure

Written by cassandra tribe

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Air Pressure Map (stuffintheair.com)

The easiest way to calculate the barometric pressure of an area is to use a high quality, calibrated barometer. If you do not own one of these expensive items, here is a way you can calculate the pressure using a few simple pieces of data you can get from your local weather station. By following these steps, you will be able to substitute values into the standard equation to calculate barometric pressure and solve it to find the pressure for any given geographical area.

Skill level:

Easy

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Things you need

Scientific calculator

Temperature of the location

Height of the location above sea level

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Instructions

1

Convert the temperature of the location you are trying to find the barometric pressure for into Kelvins. To find what the temperature in Celsius is in Kelvins, take the number of degrees in Celsius and add 273.15. So, 20 degrees Celsius (C) + 273.15 = 293.15. Therefore, 20 degrees Celsius is equal to 293.15 Kelvin(K). The formula looks like this: K = C + 273.15 To find what a location's temperature in Fahrenheit is in Kelvins, convert the Fahrenheit temperature to Celsius. You can do this by subtracting 32 from the Fahrenheit (F) temperature and then divide by 1.8. The formula looks like this: C = (F -- 32) / 1.8

2

Determine what the "standard temperature lapse rate" is for the purpose of solving the equation. The standard temperature lapse rate is measured at (- 2) °C per 1000m. Divide the height above sea level (which should be in meters) by 1,000 and then multiply by (-2). If the height above sea level is calculated in feet, multiply the height in feet by 0.3048 to find the height above sea level in meters.

3

Solve the equation part by part in the next steps. Add the standard temperature plus the standard temperature lapse rate. The solution to that equation is {T}. This is what the equation will look like: {standard temperature} + {standard temperature lapse rate} = T Remember, the equation you are going to use requires the temperature to be in Kelvin, not Celsius or Fahrenheit.

4

Subtract the height of layer b from the height above sea level. The height of layer b represents one of the six designated layers of the Earth's atmosphere. The only one you need to be concerned with is the troposphere, or the first layer, which has a value of 11,000 meters. Your equation will then look like this, with Q being the solution: {height above sea level in meters} -- 11,000m = Q

5

Multiply T times Q. Once you have done this, you are going to write a fraction. The solution you found in multiplying T times Q is the denominator of the fraction. The standard temperature in Kelvins will be the numerator.

6

Raise the fraction you created to an exponential power. The power is in a fraction form containing two equations. Solve each equation in the exponent separately before you write them next to the fraction. In the denominator of the fraction, solve the following equation: {8.31432} times {standard temperature lapse rate}

The first number (8.31432) is the Universal Gas Constant for air. The second number is the conversion you arrived at in Step 2 for your standard temperature lapse rate. The solution to this problem becomes the denominator of your exponent fraction. The numerator of your exponent fraction is derived from the following equation: {9.80665} times {0.0289644} The first number in that equation represents the gravitational acceleration of the earth. The second number is molar mass of Earth's air.

7

Raise the original fraction to the exponent that you have found. The equation will read like this: The standard temperature in Kelvins over T times Q raised to the power of {0.28404373} over {8.31432} times {standard temperature lapse rate} Solve this equation for the value of N.

8

Calculate the final equation you have to solve to find the barometric pressure. It is N times the static pressure. In this instance of finding the barometric pressure for a geographic location, the static pressure will always be {22632.1}. So, the barometric pressure is equal to N times {22632.1}.

Tips and warnings

If you are not sure what temperature you wish to arrive at the barometric pressure for in a specific location, take a long range average of readings to determine an average pressure.

The steps here solve one permutation of the equation for barometric pressure. There are other equations available that take into account variables that may exist in your inquiry.

There are many sites on the Internet that will show you how to use a much easier calculation to figure out the barometric pressure for a particular location. The equation they are using is for the Ideal Gas Law. While you can calculate atmospheric pressure using the Ideal Gas Law, you cannot use it to discover the pressure in an atmosphere that contains liquid, such as Earth's natural atmosphere. Calculating using the Ideal Gas Law is easier, but your answer will be incorrect if you are trying to determine the barometric pressure of a geographic location.